In such devices as heat exchangers and the like, a plurality of tubes, often arranged in incoming and outgoing tube bundles, is used for providing a flow path for working fluids. Relatively thin tube sheets have been used to fix the location of tube bundles. Pressurized working fluid is introduced through an inlet pipe emptying into an inlet inventory chamber, which fluid then flows through the tubes, wherein indirect heat exchange takes place with a secondary fluid; thereafter the fluid empties into an outlet inventory chamber, and exits through an outlet pipe. Typically, a hemispherical head arrangement has been associated with such heat exchange devices, a characteristic of which is a relatively large inventory chamber for the working fluid. Due to this large chamber, a considerable volume of working fluid is contained in the heat exchange device at any time. The shortcoming of the header arrangement is that working fluid, especially in the case of nuclear heat exchangers, is quite expensive, and therefore, associated material costs are high. For example, heavy water, D.sub.2 O, one popular working fluid, is now priced at approximately $2,000 per cubic foot.